In recent years, in an image forming apparatus (e.g., MFP: multi-function peripheral), a sheet finishing apparatus is arranged adjacent to a post stage of the MFP in order to apply finishing to a sheet subjected to image formation. The sheet finishing apparatus is also called finisher. The sheet finishing apparatus applies punching and stapling to a sheet sent from the MFP. The finisher includes, in order to punch the sheet, a punching unit including plural punching blades. The punching blades ascend and descend according to the rotation of a punch motor.
A sheet sent from the image forming apparatus is sometimes skewed. If the sheet is punched while remaining skewed, a problem occurs in filing the sheet. Therefore, a skew correcting device is provided to correct the skew of the sheet before the sheet is punched.
After punching the sheet, the punching blades ascend to a standby position (a home position) spaced away from the sheet surface. The punching blades are moved in a direction orthogonal to a conveying direction of the sheet, whereby a sensor, which detects the lateral ends of the conveyed sheet, detects the size of the conveyed sheet.
In the related art, there is a drawback in that, if the sheet to be punched is relatively thick, the punch motor needs a large electric current or it is necessary to provide the punch motor having a large driving force.